A general manufacturing process of an automobile includes welding various structural parts such as a vehicle body panel molded by a press to complete an integrated vehicle body. Surfaces of each part are painted and unpainted for the completed vehicle body. A powertrain, suspension system, steering system, braking system, etc., are assembled, and then a door, a trunk lead, and a hood are assembled.
In the process of assembling the vehicle body, spot welding which is resistance welding is mainly applied to bond vehicle body parts, such as a roof, a pillar, a side panel, and a vehicle door flange. The spot welding melts contact portions by heat generated at the contact portions due to electric resistance and bonds the contact portions by applying a pressure thereto.